The invention concerns a method and a device for noise reduction in a pulse-width-controlled image-display device.
For example, such a method and such a device are used in plasma displays, which will complement or replace the color tubes still used in high-quality television equipment at the present time. In connection with color tubes, the user of high-quality television equipment has been accustomed to flicker-free reproduction since the end of the 80s due to the 100 Hz technology.
A plasma display is known from the journal, Radio Fernsehen Elektronik RFE, No. 2, 1997, pages 18–20, which consists of two glass plates with electrodes arranged in a matrix, between which a noble gas mixture is located. The image information in plasma displays is not represented by lines, as in the case of the cathode-ray tubes, but as a whole image. Since, in a plasma display, the individual image points cannot be turned on and off individually at arbitrary times, the activation of the image point must occur in an activation pass for the entire display.
The start-up of a plasma display occurs in several phases: an addressing- or initialization phase, a holding- or activation phase and a turn-off phase.
In the addressing- or initialization phase, all cells of the plasma display which are to be activated in the subsequent holding- or activation phase are precharged. In the last step, the turn-off phase, the precharged cells are discharged and the image information is turned off.
The time interval available for the representation of a TV image is divided into time intervals of different duration or different weighting, while a predetermined activation sequence is chosen as a function of the brightness of a particular image point. This corresponds to lighting up of the particular image point once or several times during the image presentation in the available time interval, where a predetermined time duration is assigned to each lighting up process.
Such known plasma displays are sold on the market, for example, by the companies Fujitsu and NEC.
From DE 198 33 597 A1, a method and a device are known for reduction of flicker in pulse-width-controlled image-display devices, especially in a color plasma display. Such a color plasma display serves, for example, for the representation of TV images. The color plasma display is controlled with a pulse-width modulator, whereas, for triggering, the duration of a TV image is divided into a number of partial images or partial time intervals, which are shown sequentially. In order to reduce flicker, especially to reduce a 50 Hz flicker, the sequence of the partial time intervals and/or activation sequences of the partial time intervals is predetermined so that the flickering of the images to be presented is minimum.
Furthermore, a motion-detector-dependent change of the sequence of time intervals is known. When motion is present, the sequence of time intervals is chosen so that motion artifacts are avoided. Otherwise the sequence of the time intervals is such that 50 Hz flicker disturbances are reduced.
Furthermore, it is already known in connection with plasma displays that one can determine the brightness of an image to be represented, derive a maximum permissible illumination duration for each partial time interval of the image to be represented from the determined brightness value and change the maximum permissible illumination duration for each of the partial time intervals in case of change of the detected brightness value. This change is done in such a way that, at a determined dark image content or low brightness value, the maximum permissible illumination duration is increased in each of the partial time intervals by the same time duration. However, on the other hand, if it is found that, in the brightness determination of the image to be represented, an overall bright image content is present, then the maximum permissible illumination duration of each partial time interval is reduced by a duration which is the same for all partial time intervals.
A disadvantage of this procedure consists in the fact that the contrast of the image to be produced is reduced, because, if the brightness of the image to be represented is high, the time duration for the representation of bright image components is reduced, and, if the brightness of the image to be represented is low, dark gray image components are represented as light gray, because these are pulled up, that is, are lit up longer as a result of the application of a constant offset described above.
Furthermore, a method is known from German Patent Application 100 09 858 for improving the contrast of a pulse-width-controlled image display device. In this method, a change of the maximum permissible illumination duration for the partial time intervals is carried out in such a way that the maximum permissible illumination duration is subjected to smaller changes for partial time intervals with lower value than the maximum permissible illumination duration for partial time intervals with a higher value.